the UnsupportedOperationException in RMI

I worked a simple program But when you run the client at the command This error appears

 HelloClient exception:  java.lang.UnsupportedOperationException: Not supported yet.

this my coded

Interface class

    import java.rmi.*;

    public interface HelloInterface extends Remote { 

     public String say() throws RemoteException;


   }

implement class

           import java.rmi.RemoteException;
           import java.rmi.server.UnicastRemoteObject;

         /** 
           *
           * @author x
           */
   public class HelloServerImpl extends UnicastRemoteObject implements HelloInterface {

   private String message; 

   public HelloServerImpl(String msg)throws RemoteException{
   message = msg;
   }


@Override
public String say() throws RemoteException {
    throw new UnsupportedOperationException("Not supported yet.");
}




 }

Server class

     import java.rmi.Naming;

       /**
        *
        * @author x
        */
       public class HelloServer {
        public static void main (String []args ){
          try {
        Naming.rebind("HELLOSERVER", new HelloServerImpl("Hello word"));
        System.out.println("Hello Server is ready.");
    } catch (Exception ex) {
        System.out.println("Hello server failed: "+ ex);
    }


    }
    }

Client class

          import java.rmi.Naming;

         /**
          *
          * @author x
          */
             public class HelloClient {
          public static void main(String[]args){

        HelloInterface hello;
        String url = "rmi://localhost/HELLOSERVER";


         try {
        hello = (HelloInterface)Naming.lookup(url);
        System.out.println(hello.say());
    } catch (Exception ex) {
       System.err.println("HelloClient exception:  " + ex);
    }

    }
   }

I am prepared to write the steps but still the same error

why??

Well you wrote this yourself:

@Override
public String say() throws RemoteException {
    throw new UnsupportedOperationException("Not supported yet.");
}

Of course it throws an exception. Try to actually return a string:

@Override
public String say() throws RemoteException {
    return "hello";
}